The presence of large deposits of oil shale in the semi-arid, high plateau region of the western United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising a marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products, including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either mining the kerogen-bearing shale and processing the shale on the surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale, after kerogen decomposition, contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded to form a fragmented permeable mass of particles containing oil shale. The fragmented mass is ignited near the top of the retort to establish a combustion zone. An oxygen-supplying gas is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented permeable mass of particles in the retort. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting zone. The retorting zone moves from the top to the bottom of the retort ahead of the combustion zone and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S. Pat. No. 3,661,423 to Donald E. Garrett which is incorporated herein by this reference.
Examples of techniques used for forming in situ oil shale retorts are described in U.S. Pat. Nos. 4,043,597 to French, 4,043,598 to French et al, and 4,192,554 to Ricketts. According to these patents, at least two voids vertically spaced apart from each other are excavated in a subterranean formation leaving zones of unfragmented formation between adjacent voids. Explosive is placed in blasting holes formed in the zones of unfragmented formation. The explosive is then detonated to expand formation toward the voids to form a fragmented mass having a void volume about equal to the void volume of the initial voids. U.S. Pat. Nos. 4,043,597, 4,043,598, and 4,192,554 are incorporated herein by this reference.
A fragmented mass of formation particles formed in a retort preferably has a reasonably uniformly distributed void fraction and permeability so that gases can flow relatively uniformly through the retort during retorting operations. This avoids gas bypassing of portions of the fragmented mass, as can occur if there is channelling due to non-uniform permeability and, thus, enhances the yield of liquid and gaseous products from the retort.
When formation is prepared for explosive expansion toward one or more voids in a subterranean formation for forming a fragmented mass in a retort, it sometimes is desirable to place more than one explosive charge into a single long blasthole. Such charges are spaced apart from each other by stemming with inert materials such as sand or gravel or the like. In some instances, it is desirable to detonate each of these separate charges at a different time in a single round of explosions coordinated with detonations of explosive charges in other blastholes in the formation. In such a blast, it is important that each charge is detonated and that such a detonation is at the proper time in the sequence so that the fragmented mass formed has the desired uniformity of void fraction distribution and permeability.
One problem caused by using a time delay method of blasting is that ground movement and/or airborne rock fragments ejected from a previous explosion can sever explosive initiating means. The initiating means, for example, can be trunk lines containing tie-up systems of detonating cord and time delay devices. Severing a trunk line can result in cutoff of a blasthole or blastholes serviced by the severed trunk line where the explosive in the blasthole is not initiated due to the severance. Lack of initiation of explosive in the blastholes causes formation in the area to remain unfragmented, resulting in an uneven distribution of void fraction or permeability of the fragmented mass in the retort.
In order to substantially decrease the probability of having a cut off blasthole, it is desirable to initiate all of the explosive trains downhole at the same time prior to the first explosions in a round of time delay explosions. Explosive trains include initiating devices such as detonating time delays and their associated detonating cords.
In one method of expanding unfragmented formation in a single round of time delay explosions, a plurality of long vertical blastholes are drilled into a subterranean formation from a void space above the formation. A first explosive charge is placed into the bottom of the blasthole, stemming is placed above the first charge, and a second charge is placed into the blasthole above the stemming. Associated with each explosive charge is at least one primer and an associated detonating time delay device. In many instances, a plurality of primers are used for each charge to provide redundancy.
In an exemplary technique, two primers are embedded in the first explosive charge at the bottom of the blasthole. If desired, a pair of time delay detonators may be placed in each primer. The detonating time delay device embedded in each primer is connected to a separate detonating cord lead, each of which extends up the blasthole, through the second explosive charge, and out the top of the blasthole into the void space above. Additionally, one or more, and in this instance two, primers and their associated time delay devices are embedded in the second explosive charge. A detonating cord lead is connected to each of the two detonating time delay devices associated with the second charge and each lead extends upwardly from the second explosive charge and out the top of the blasthole into the void space. In this instance, therefore, four detonating cord leads associated with the four time delay devices are in the blasthole and pass through the second explosive charge.
All four detonating cord leads are initiated at about the same time so that, in turn, the time delay device associated with each respective lead is initiated downhole at about the same time as each other time delay. Thus, energy from the detonation of all four leads is transmitted about simultaneously to the second explosive charge. The amount of energy released can be sufficient to cause the second charge to detonate prematurely. Such premature detonation can cause an uneven distribution of void fraction or permeability in the fragmented mass which can result in gas channelling and bypassing of portions of the fragmented mass as is described above.
In addition, detonation of the first explosive charge in such a blasthole can be less reliable than desired. For example, during loading of the stemming and second explosive charge, or when the first charge slumps or settles in the bottom of the blasthole, the detonating cords or other explosive initiating leads, such as electrical leads used with electrically initiated blasting caps, that extend from the first charge can be severed. In this case, the first charge will not detonate and, thus, the fragmented mass formed in the retort may not be as uniformly permeable as desired.
Loading explosive into blastholes also becomes increasingly more difficult as the number of downlines, i.e , explosive initiating leads, increases due to hangup and tangled lines and the like.
It is, therefore, desirable to provide a method of enhanced reliability for explosively expanding unfragmented formation when using long blastholes containing more than one explosive charge.